1. Field of the Invention
The invention relates to a sheet conveying apparatus, in particular, an apparatus for aligning a sheet.
2. Description of the Related Art
Heretofore, in an image forming apparatus such as a copying machine or a printer, a toner image formed on an image bearing member such as a photosensitive drum in an image forming portion has been transferred to a sheet, and thereafter has been fixed on the sheet by fixing means such as heat fixing means or pressure fixing means to thereby effect image formation.
Such an image forming apparatus is provided with a sheet conveying apparatus for conveying the sheet to the image forming portion. Such a sheet conveying apparatus is provided with a sheet aligning portion for correcting the skew feed or position of the sheet being conveyed, and the skew feed or position of the sheet is corrected by this sheet aligning portion to thereby prevent the deviation of the image formed on the sheet.
As a technique adopted in such a sheet aligning portion, there is one described, for example, in Japanese Patent Application Laid-open No. S63-225052, which corrects the attitude of a sheet by contacting the sheet being conveyed against a stopper provided for forward and backward movement in a sheet conveying path.
Also, there is such a technique as shown in Japanese Patent Application Laid-open No. H11-189355 wherein a sheet is conveyed while a side edge of the sheet contacted against a reference guide along a sheet conveying direction by a diagonal feed roller, whereby the attitude of the sheet is corrected with the side edge of the sheet as the reference.
Now, cut paper is often used in the image forming apparatus and as this cut paper, there is circulated in the market one which is low in the accuracy of the right angle between the leading edge and the side edge orthogonal to each other, or irregular in the degree of the right angle. Further, in other sheets than the cut paper, there is a case where a reduction in or the irregularity of the accuracy of the right angle occurs due to the shrinkage by drying or the like.
Accordingly, when in the conventional sheet conveying apparatus, such a technique as described above which corrects the attitude of the sheet with the leading edge or the side edge of the sheet as the reference, the following problem arises. That is, when use is made of a sheet which is low in the accuracy of the right angle between the leading edge and the side edge, if the attitude of the sheet is corrected, for example, with the side edge thereof as the reference, the deviation of an image forming position will occur relative to the leading edge because the leading edge is not at a right angle with respect to the side edge.
Also, when processing such as bookbinding or cutting is to be effected on a sheet, if the processing such as bookbinding or cutting is effected with the leading edge of the sheet as the reference after sheet alignment is effected, for example, with a side edge of the sheet as the reference, the inclination of an image or the irregularity of an image position will occur because the leading edge is not at a right angle with respect to the side edge.
That is, in a case where the accuracy of the right angle of the sheet is low or in a case where the degree of the right angle of the sheet is irregular, when an image is to be formed on the sheet, deviation occurs to the image forming position, and during the processing of the sheet, the irregularity or the like of an image position occurs.